Desulfurization by selective adsorption with a crystalline zeolitic molecular sieve

ABSTRACT

Sulfur compounds are removed from liquid hydrocarbon streams by selective adsorption on zeolitic molecular sieves and periodic desorption therefrom using a hot purge gas having a high water content. Regeneration of the adsorbent is carried out only to the degree that less than about 5 and more than about 2 weight percent water remains adsorbed on the zeolite.

United States Patent [72] Inventors Philip Ilain Turnock Katonah; MaxNai Yuen Lee, Yorktown Heights; Krlshan Dayal Manchanda, Scarsdale, allof N.Y.

[21] Appl. No. 866,484

[22] Filed Oct. 15, 1969 [45] Patented Nov. 16, 1971 [73] Assignee UnionCarbide Corporation New York, N.Y.

[54] DESULFURIZATION BY SELECTIVE ADSORPTION WITH A CRYSTALLINE ZEOLITICMOLECULAR SIEVE 3 Claims, 4 Drawing Figs.

[52] U.S.Cl 208/245,

[51] Int.Cl ....C10g25/04 [50] Field of Search 208/208,

Pl/RGE GAS SWEETENED PRODUCT Primary Examiner-Delbert E. Gantz AssistantExaminerG. J. Crasanakis Attorneys-Thomas l. OBrien and Richard G.Miller ABSTRACT: Sulfur compounds are removed from liquid hydrocarbonstreams by selective adsorption on zeolitic molecular sieves andperiodic desorption therefrom using a hot purge gas having a high watercontent. Regeneration of the adsorbent is carried out only to the degreethat less than about 5 and more than about 2 weight percent waterremains adsorbed on the zeolite.

PATENTEUNHV 16 I971 8,620,969

SHEET 2 OF 2 PHIL IP H. TURNOC KRISHAN 0. MANCHANDA MAX IV. V. LEE

BY flmlm 2/ nd/m ATTORNEY DESULFURIZATION BY SELECTIVE ABSORPTION WITH ACRYSTALLINE ZEOLITIC MOLECULAR SIEVE This application relates in generalto an improved process for sweetening hydrocarbon liquids and moreparticularly to a process for removing sulfur compounds from naturalgasoline which employs as the selective absorbent for thesulfur-containing impurities a zeolitic molecular sieve dehydrated onlyto the extent that it contains a residual loading of water of from about2 to about 5 weight percent.

The removal of sulfur compounds, particularly hydrogen sulfide and thealkyl mercaptans from hydrocarbon streams is desirable for many reasons,depending in part upon the intended use of the final sweetened product.Since a very large percentage of the lighter hydrocarbons are ultimatelyused as fuel per se, the presence of sulfur compounds is objectionablebecause of the unpleasant odor imparted and the air pollution resultingfrom the combustion. When used as fuels for internal-combustion engines,the sulfur compounds are deleterious to the effectiveness of knownoctane improvers such as tetraethyllead. The heavier hydrocarbons arelargely subjected to hydrocarbon conversion processes in which theconversion catalysts are, as a rule, highly susceptible to poisoning bysulfur compounds.

Several methods for sweetening hydrocarbon streams have been proposedand utilized in the past, including both chemical and physicaltechniques. The chemical processes have involved purely chemicalreactions such as scrubbing with monoethanolamine or countercurrentextraction using a hot potassium carbonate solution, and chemisorptionmethods in which iron oxide sponge perferentially collects the sulfurcompound on its surface.

Selective physical absorption of sulfur impurities on crystallinezeolitic molecular sieves is a more recently proposed technique and isnow perhaps the most widely used method. Both liquid phase and vaporphase processes have been developed, with liquid phase operation beingpreferred for use with olefinic streams and low-boiling paraffinichydrocarbon streams.

A liquid phase hydrocarbon sweetening process comprises passing asulfur-containing hydrocarbon fraction such as a stabilized or a fullrange natural gasoline through a bed of a molecular sieve adsorbenthaving a pore size large enough to adsorb the sulfur impurities,recovering the nonadsorbed effluent hydrocarbon until a desired degreeof loading of the adsorbent with sulfur-containing impurities isobtained, and thereafter purging the adsorbent mass of hydrocarbon andregenerating the adsorbent by desorbing the sulfur-containing compoundstherefrom.

The adsorbent regenerating operation is conventionally a thermal swingor combined thermal and pressure swing-type in which the heat input issupplied by a hot gas relatively inert toward the hydrocarbons, themolecular sieve adsorbent and the sulfur-containing adsorbate. Naturalgas is ideally suited for use in purging and adsorbent regeneration,provided that it can subsequently be utilized in situ as fuel wherein itconstitutes an economic balance against its relatively high cost.Frequently, however, the sweetening operation requires more natural gasfor thermal-swing regeneration than can be advantageously consumed asfuel, and in effect, this constitutes an inadequacy of regeneration gas.The result is a serious impediment to successful design and operation ofsweetening processes, especially when desulfurization is carried out ata location remote from the refinery, as is frequently the case.

One potential system for generation of an inert atmosphere combines fueland air in nearly stoichiometric proportions to obtain a combustionproduct devoid of oxygen. The combustion products, consisting primarilyof carbon dioxide and water in nitrogen, amount to about nine times thevolume of natural gas consumed. The generated gases ordinarily undergosubsequent treatments to reduce the levels of carbon dioxide and water.Thus, the cost of any such inert gas supply is generally influencedconsiderably by the required purity and moisture level in the gas. Theinvestment in equipment to supply low-pressure inert gas at a -40 F. dewpoint can readily amount to twice that required for generation of acomparable gas supply at a F. dew point.

In accordance with the present invention it has been surprisingly foundthat the presence of relatively large amounts of residual adsorbed wateron large pore zeolitic molecular sieve adsorbents does not diminishtheir selectivity nor unduly diminish their capacity for adsorbingsulfur-containing impurities from liquid hydrocarbon streams. ln markedcontrast to this finding, it is well known that in vaporphase-sweetening processes, the presence of even relatively smallamounts of water on the molecular sieve adsorbent has such an adverseeffect on its selectivity and capacity as to render the operationcommercially unfeasible. For example, the dynamic equilibrium capacity(450 p.s.i.g., 86 F.) of fully dehydrated sodium zeolite X (i.e., about0.1 weight-percent residual H O) for H 8 from a natural gas containing540 p.p.m. (volume) H S is es sentially the same as when the H 8concentration in liquid bu tane is about 1,050 p.p.m. (volume). With gasphase adsorption under the same conditions of temperature and pressure,the presence of a residual H O loading on the sodium zeolite X of only1.8 weight-percent causes a decrease in the equilibriurn adsorptioncapacity for H 5 of 48 percent. In contrast, sodium zeolite X containingas much as 4.1 weight-percent residual water loading has been found toundergo a dynamic equilibrium capacity decrease for H 8 from a liquidphase system of only 30 percent with comparable H S concentrations. Thepresent discovery thus makes it now possible to carry out a liquidhydrocarbon-sweetening process in which the thermal-swing desorption ofthe sulfur-containing compounds from the molecular sieve adsorbent isaccomplished using a purge gas having a high water content. It is evenpossible to utilize steam as the purge gas and this is a preferredembodiment of the regeneration procedure of the present invention.

The hydrocarbon stocks suitably treated in accordance with the presentinvention are not critical with respect to origin, constituent molecularspecies or relative proportions of the molecular species within thefeedstock. Thus, the stocks can be hydrocarbonaceous product resultingfrom the destructive hydrogenation of coal or they can be obtained fromdeposits of natural gas or petroleum. Sulfur-containing condensates fromnatural gas, i.e., the LPG compositions rich in propane and butanes arewell suited to the present process as are natural gasolines andrelatively light petroleum fractions boiling between about 44 to aboutF. which are mostly comprised of C to C hydrocarbons. Moreover, liquidor liquefiable olefin or olefin-containing streams, such as those usedin alkylation processes, containing propylene, butylene, amylene and thelike are also suitably employed.

The sulfur-containing impurity present in the hydrocarbon feedstockscomprises at least one but ordinarily a mixture of two or more ofhydrogen sulfide, the mercaptans such as ethyl mercaptan, n-propylmercaptan, isopropyl mercaptan, n-butyl mercaptan, isobutyl mercaptan,t-butyl mercaptan, and the isomeric forms of amyl and hexyl mercaptan,the heterocyclic sulfur compounds such as thiophene and 1,2-dithiol, thearomatic mercaptans exemplified by phenyl mercaptan, organic sulfidesand disulfides generally and carbonyl sulfide.

The adsorbent materials employed in the present process are the naturalor synthetically prepared crystalline zeolitic aluminosilicates commonlyreferred to as molecular sieves or zeolitic molecular sieves. Unlikemost adsorbents which have a variety of pores of different dimensions,molecular sieve adsorbents are characterized, in part, by having poresof uniform dimension, and thus the particular species selected must havepore diameters large enough to permit passage therethrough of the sulfurcompounds present in the hydrocarbon feedstock. Molecular sieves havingpores with an apparent minimum dimension of at least 3,8 angstrom unitshave been found satisfactory when the sulfur compound impurity which isto be adsorbed is hydrogen sulfide. For normal mercaptans having lessthan seven carbon atoms, the apparent pore size should be at least about4.6 angstrom units. The sulfur compounds of larger molecular dimensionssuch as isopropyl mercaptan, isobutyl mercaptan, t-butyl mercaptan, theisomeric form of amyl and hexyl mercaptan, and the heterocyclic sulfurcompounds exemplified by thiophene as well as the aromatic mercaptansexemplified by phenyl mercaptan require the use of a zeolitic molecularsieve having apparent pore openings of at least about 6 angstrom units.Since it is uncommon that a sour hydrocarbon feedstock contains nosulfur compounds of large molecular size, it is preferred that themolecular sieve employed have an apparent pore diameter of at least 8angstroms and more preferably at least 10 angstroms.

The term apparent pore size as used herein may be defined as the maximumcritical dimension of the molecular species which is adsorbed by thezeolitic molecular sieve in question under normal conditions. Theapparent pore size will always be larger than the effective porediameter, which may be defined as the free diameter of the appropriatesilicate ring in the zeolite structure.

Among the naturally occurring zeolitic molecular sieves suitable for usein the present invention include mordenite and chabazite both having anapparent pore size of about 4 angstrom units, erionite having anapparent pore size of about angstrom units, and faujasite having a poresize of about angstroms. The natural materials are adequately describedin the chemical art. The preferred synthetic crystalline zeoliticmolecular sieves include zeolites X, Y, L and Q, and large poremordenites. Zeolite L has an apparent pore size of about 10 angstroms,and is described and claimed in U.S. Pat. No. 3,216,789. Zeolite X hasan apparent pore size of about 10 angstroms, and is described andclaimed in U.S. Pat. No. 2,882,224, having issued Apr. l4, 1959 to R. M.Milton. Zeolite Y has an apparent pore size of about 10 angstroms, andis described and claimed in U.S. Pat. No. 3,130,007.

Zeolite Q. is described in pending U.S. application Ser. No.

655,318, filed July 24, 1967. Zeolite L is described in U.S. Pat. No.3,216,789, and the preparation of a large pore synthetic mordenite isdisclosed in U.S. Pat. No. 3,436,174, issued Apr. 1, 1969 to L. B. Sand.

The pore size of the zeolitic molecular sieves may be varied byemploying different metal cation. For example, sodium zeolite A (U.S.Pat. No. 2,882,243) has an apparent pore size of about 4 angstrom units,whereas calcium zeolite A has an apparent pore size of about 5 angstromunits.

The zeolites occur as agglomerates of fine crystals or are synthesizedas fine powders and are preferably tableted or pelletized for largescale adsorption uses. Pelletizing methods are known which are verysatisfactory because the sorptive character of the zeolite, both withregard to selectivity and capacity, remains essentially unchanged. Manysuitable inert binder materials or compositions are well known in theart including clays, refractory metal oxides and alkali metal silicates,if it is desired to utilize the adsorbents in agglomerated form. Ingeneral, the individual molecular sieve crystals are quite small (of theorder of 10 microns) and hence in fixed bed operation, at least, it isadvantageous to agglomerate the crystals into beads, pellets, extrudateforms, etc., either with or without added binder material.

The method by which the sour hydrocarbon stream and the molecular sieveadsorbent are brought into contact in order to selectively adsorb thesulfur compounds therefrom is not a critical part of this invention. Forexample, the operation can involve one or more fixed adsorbent beds,moving beds, slurry beds or combinations thereof and the hydrocarboncharge material can flow in direct, countercurrent or cocurrent contactwith the molecular sieve. The use of fixed adsorbent beds is mostfrequently used in hydrocarbon stream sweetening, and incorporates thefollowing processing steps in conventional sequence: (a) adsorption ofundesirable sulfur impurities from the liquid hydrocarbon stream; (b)displacement of liquid from the bed voids; (c) volatilization ofoccluded liquid; (d) regeneration of the molecular sieve; (e) cooling ofthe adsorbent; and-(f) filling the bed for further feed purification.

With specific reference to a typical method of operation in P10. 1, twobeds, 10 and 11 of crystalline zeolitic molecular sieve material areprovided and piped in parallel flow relation so that when one bed is onthe adsorption stroke, the other bed is being regenerated by purging andcooldown. In this manner, a continuous supply of sulfurcompound-depleted hydrocarbon liquid is available for consumption. If acontinuous supply is not required, it may be preferable to employ asingle bed of zeolitic molecular sieve adsorbent, and provide a productliquid supply during the intermittent periods when such bed is onadsorption stroke.

The sour sulfur compound-containing liquid hydrocarbon feed stream isintroduced through conduit 12, preferably at ambient temperaturealthough there is no sharply defined critical region in this respect.Choice of the optimum temperature depends on an economic balance betweensavings in zeolitic molecular sieve material by virtue of higheradsorptive capacities at lower temperatures, and the cost of heatexchangers to obtain the lower temperature. Viscosity may also be alimitation on heavy naphtha streams. With regard to feed pres sure, theonly limitation in this respect is that the pressure be sufficientlyhigh to keep the feed in the liquid phase throughout the adsorber bed toavoid internal flashing with consequent poor contact with the molecularsieve and attrition of particles.

It has been found that the adsorption step may be efficiently performedwith feed liquid superficial linear velocities of 0.1 to 20 feet perminute, and preferably between 1 and 10 feet per minute. The reasons forthese criticalities are as follows: at low superficial linearvelocities, a thin film of liquid exists on the exterior surface of eachzeolitic molecular sieve particle, primarily due to the viscosity of thehydrocarbon feed liquid. The sulfur compound must pass through this filmfor flow through the pores and into the inner cage of the crystalstructure for adsorption thereby, and such passage is resisted by thefilm so as to decrease the adsorption rate. As the feed liquid velocityincreases the thickness of the liquid film decreases, thereby reducingthe external film resistance and increasing the rate of adsorption.Finally, as the superficial linear velocity is further increased, theliquid film is substantially eliminated and the efficiency of theadsorption step becomes primarily dependent on contract time between thesulfur compound containing feed stream and the zeolitic molecular sieve.That is, sufficient time must be provided for the sulfur compound totransfer from the feed stream to the molecular sieve, and higher feedliquid velocities will of course reduce such contact time. Still anothercharacteristic of higher superficial linear velocities is increasedpumping costs. It has been found that a superficial linear velocity ofbelow 0.1 feet per minute produces an excessively high external filmresistance, while a velocity of over 20 feet per minute does not permitsufficient contact time for high adsorptive efficiency. Within thisbroad range, the adverse effect of the external liquid film isessentially eliminated at a velocity above 1 foot per minute. Also, therequired bed length becomes unduly long when the superficial linearvelocity is above 10 feet per minute, due to the reduced contact timeand lower adsorptive efficiency.

The process will efficiently handle feed streams containing minutetraces of sulfur on the order of 5X10""" weight-per cent up to thosecontaining 2 weight-percent sulfur compounds. This process isparticularly advantageous in the sulfur trace concentrations because ofthe relatively high sulfur loadings attainable on crystalline zeoliticmolecular sieves.

The upper limit of 2 weight-percent sulfur compound con centration inthe feed liquid is based on the fact that liquid phase adsorptionbecomes impractical when the sulfur concentration exceeds this levelsince the duration of the adsorp tion step becomes relatively short ascompared with the necessary duration of the desorption and cooldownsteps. Stated in another way, a prohibitively large adsorption bed wouldbe required to obtain an adsorption step of specified duration if thesulfur concentration exceeds 2 weight-percent.

The sulfur-containing liquid hydrocarbon feed stream is directed fromconduit 12 to communicating conduit 13 joining at its opposite end withthe inlet and upper end of first zeolitic molecular sieve bed 10.Conduit 13 also contains flow control valves 14 and 15 arranged in aseries relationship.

Discharge conduit 16 communicates with the lower end of bed 10, andcontains flow control valves 17 and 18 arranged in series relationship,and valve 17 is at least partially closed during the initial part of theadsorption stroke, so that the bed may be filled with liquid hydrocarbonfeed. Control by means of valves 14 and 15 can be exercised over therate at which the liquid is introduced during the filling step to avoidchanneling of the liquid downwardly through the bed as this may resultin premature vaporization in the lower regions of the bed with resultantpressure buildup and violent movement of the bed. Flow distributingdevices as are well understood by those skilled in the art, may beemployed to prevent such channel- When the zeolitic molecular sieve bed10 is filled with liquid, the withdrawal of purified liquid hydrocarbonproduct from the lower end of the bed is begun through conduit 16 andcontrol valves 17 and 18 therein. The sweetened liquid hydrocarbonproduct stream is discharged from the system through communicatingconduit 19. Simultaneously, the sour liquid hydrocarbon feed stream isintroduced through valves 14 and at a rate sufficient to maintain bed 10completely filled with liquid. As the adsorption step or stroke iscontinued, the sulfur compounds are selectively adsorbed by themolecular sieve in a downwardly advancing zone, in which zone thepreviously adsorbed hydrocarbon is being displaced by the sulfurcompounds. The liquid removal step is downward to afford superiordrainage, and thus improve the efficiency of the adsorption step.

The adsorption step may be continued until the appearance of sulfurcompounds in the product indicates that the capacity of the molecularsieve has been attained. At this point, however, the free spaces in thebed not occupied by molecular sieve material are filled with sour liquidhydrocarbon which must either be sent to a fresh molecular sieve bed ordischarged. Preferably, valve 15 through which the sour feed enters bed10 is closed when the latter has only sufficient remaining adsorptivecapacity to remove the sulfur compound contained in the liquid remainingin the free spaces or interstices of the bed.

At this point, valves 17 and 18 are closed and the sour liquidhydrocarbon feed stream is diverted from conduit 12 throughcommunicating conduit 20 to second zeolitic molecular sieve bed 11 whichhas previously been desorbed and recooled. The depressurization ordraining of first bed 10 should be carried on gradually to preventexcessive flashing, movement of the pellets and attrition. A 15-minuteperiod for the blowdown step has been found satisfactory. 1n thedesorption step, a hot purge gas is supplied to conduit 21 at atemperature preferably between 450 and 700 F., the purge gas beingnonoxidizing with respect to either sulfur or the hydrocarbons present,and containing moisture to a dew point level of +10 to 160 F.Conventional purge gasses such as methane, hydrogen, nitrogen and carbondioxide in combination with adequate water vapor can be employed.Economic benefit is achieved by utilizing as the purge gases theoxygen-free combustion product gases of air and methane which formerlywere not used, due to their high moisture content, without a dryingoperation. A typical embodiment of the present process utilizing thistype of purge gas is incorporated into the flow diagram of FIG. 1. Intocombustion chamber 38 there is charged stoichiometric proportions ofnatural gas and air. Upon burning, the resultant composition is veryhigh in moisture content, but is essentially free of oxygen and containsCO, and nitrogen as the other principal ingredients. Since merelycooling the combustion products will result in the condensing out ofasubstantial quantity of water, the gas mixture from combustion chamber38 are passed through heat exchanger 42 and into condenser 44. Liquidwater is withdrawn from condenser 44 which is operated at a temperaturesufficiently low that the purge gas stream emerging therefrom containsmoisture at a dew point level commensurate with the temperature andpressure conditions employed in the adsorbent bed during theregeneration. The range of dew point levels for 15-100 p.s.i.a.regeneration gas is described by the following equations: 1.b. Maximumallowable dew point temperature, F.

0.4(Regeneration Temperature, F. )-l20 2. Normal operating limit on dewpoint temperature, "F.

0.4(Regeneration Temperature, F. )140 3. a. Minimum suggested dew pointtemperature, "F.

0.3(Regeneration Temperature, F.)-140 The above equations establish thefollowing limits for dew point temperatures of the purge gas at normalregeneration temperatures ranging from 500700 F.:

a. to leave about 2 weight-percent H O on the molecular sieve.

b. to leave about 5 weightpercent H O on the molecular sieve.

The purge gas is then reheated in heat exchanger 42 and thereafterthrough conduit 21 and control valve 22 therein to branch conduit 23containing valve 24. Conduit 23 joins inlet conduit 13 between valves 14and 15, and the hot purge gas is introduced therethrough to the upperend of first zeolitic molecular sieve bed 10 for downward flow andremoval of the adsorbed sulfur compound. The cooled and sulfur compoundladen purge gas is discharged from the lower end of first bed 10 throughconduit 16 containing valve 17, and directed through branch conduit 25containing valve 26 therein to discharge conduit 27 for use as desired.Since the purge gas has been selected with respect to water content inaccordance with the foregoing equations, cessation of the desorptionstep at a time when a desired amount of the sulfur-containing compoundhas been removed from the bed will result in a regenerated adsorbent bedhaving about 2 to about 5 weightpercent adsorbed water.

In a preferred embodiment, the liquid hydrocarbon held in theinterstices of the first zeolitic molecular sieve bed 10 is drainedtherefrom at the end of the adsorption step and before the previouslydescribed desorption step is initiated. This permits recovery of theinterstitial liquid and improves the efficiency of the cycle. Suchdrainage may be effected in any of several wellknown ways following theclosing of liquid hydrocarbon feed inlet valve 14. At this point a coolgas purge at temperature below about 250 F. can be used before thedesorption step, thereby effecting additional removal of the hydrocarbonholdup.

At the end of the previously described desorption step, the reactivatedfirst bed 10 is recooled by a controlled introduction of sour liquidfeed through conduit 13 into the upper end of bed 10 for downward flowtherethrough. The precooling step is conducted downwardly from the inletto the discharge end to prevent excessive temperature rises and flashingsince the downwardly advancing liquid front recools the risingconvective currents of generated vapor. To achieve this flow, valves 14,l5, l7 and 26 are opened, and valve 24 is closed. The amount ofvaporized feed can be condensed and returned to the feed, or if verysmall in quantity, can be passed to waste or flared. lt has been foundthat about 20-35 gallons of coolant are required per pounds of molecularsieve to be cooled. The coolant is preferably fed at a rate of l-4gallons per minute per square foot of bed cross section, the maximumrate being 8 gallons per square foot per minute. Cooling is continueduntil the bed is full of liquid and essentially at the temperature ofthe adsorption stroke. Valve 26 is then closed and valve 18 opened, andthe first bed 10 is placed back on the adsorption stroke.

It should be noted that the second bed ll of zeoliticmolecular sievematerial is operated in a manner analogous to that of first bed 10 sothat during the adsorption step, sour feed is introduced through conduit12 to communicating conduit having flow control valves 31 and 32arranged in a series relationship at the upper end of bed 11. Thesweetened hydrocarbon liquid is withdrawn from the lower end of secondbed 11 through conduit 33 having flow control valves 34 and 35 thereinarranged in series. During the desorption stroke, the hot purge gas isintroduced through valve 36 in branch conduit 23 communicating withconduit 20, the sulfur compound-laden purge gas discharged from thelower end of second bed 11 is removed from the system through valve 37in conduit 25.

In accordance with the present invention, it. is not necessary that apurge gas of limited water content be employed such as in the foregoingdescription. Since we have now found that molecular sieve adsorbentscontaining as much as 5 weightpercent residual adsorbed water arefeasible for use in liquid hydrocarbon sweetening processes, andmoreover since dehydration of a molecular sieve adsorbent to a watercontent of from 2 to 5 weight-percent is easily and economicallyaccomplished, we have further found that a steam purge can be used in apreferred embodiment of the present invention provided only that anadsorbent drying step is included in the regeneration procedure.

The temperature and pressure of a steam purge steam are not narrowlycritical factors, provided of course, that the usual precautions aretaken to prevent zeolite agglomerate shattering and zeolite hydrolysisat elevated temperatures. Agglomerate shattering results from severenonuniform thermal stress induced when hot activated agglomerates arecontacted with liquid water to produce high-adsorption exotherms.Hydrolytic stability varies with the silica/alumina ratio of thezeolites and with crystal lattice type, and is at least in partconcerned with metal cation replacement or removal from the zeolite.Thus, the steam purge can vary from very wet steam to superheated steamto maintaining a purge temperature of about 212 up to about 400 F.

The sweetening process using a steam purge step is in all respects thesame as exemplified hereinbefore except that at the termination of thedesorption of the sulfur compounds, the adsorbent will containsubstantially more than the permissible 5 weight-percent adsorbed water,and an adsorbent drying step must therefore be included. For thispurpose an inert gas stream such as nitrogen, hydrogen, natural gas, andthe like, which has a water dew point not greater than a valuedetermined by the expression 0.4T-140 in which T is the temperature ofthe purge gas stream in degrees Fahrenheit, can be brought into contactwith the adsorbent crystals in an amount, for a time period, and at atemperature such that dehydration of the zeolite adsorbent to a watercontent .of from 2 to 5 weight-percent is achieved. v

In FIGS. 2, 3 and 4 of the drawings are three typical drying systemswhich can be used following a steam purge desorption. Other systems ormodiffrions of these three exemplifications will be obvious to thoseskilled in the art.

In accordance with the schematic diagram of FIG. 2, the inert dry purgegas such as methane or nitrogen is introduced into a loop at line 50 andis impelled by blower Sll through heater 52 from which it emerges at atemperature of about 550 The hot gas thereafter enters cocurrently themolecular sieve bed 53 which contains the wet steam desorbed zeoliteadsorbent. After the drying gas carrying water desorbed from the zeoliteemerges from the effluent end of the bed it is passed through awater-cooled condenser 54, operated at a temperature sufficiently low tocondense out water and produce a gas stream having a dew pointtemperature of less than F. which is thereafter recyc ed along with anymakeup gas which may be required.

The system of FIG. 2 is modified in FIG. 3 by the addition of a bed 56containing any desired desiccant through which the drying gas emergingfrom condenser 54 is passed prior to being recycled. This modificationof the system of FIG. 2 permits less rigorous cooling in the condenser54 with the consequent increase in the dew point of the gas streamemerging therefrom. The desiccant bed 54 can contain an activatedmolecular sieve adsorbent, or silica gel, calcium chloride or any otheravailable desiccant material. After emerging from the desiccant bed 56the drying gas is thereafter cycled through blower 51, heater 52 andmolecular sieve bed 53 which is being dried. The modification of FIG. 3substantially decreases the drying cycle time because of the more rapidtransfer of moisture from the molecular sieve in bed 53 to the drier gasstream passing therethrough.

The drying scheme of FIG. 4 utilizes butane or other readily condensableregeneration media which is impelled by pump 57 into vaporizersuperheater 58 before entering cocurrently wet molecular sieve bed 53.The water-laden drying gas emerging from bed 53 is cooled to condensethe butane in condenser 54 and thereafter the liquid butane is separatedfrom the water it has carried from the bed 53 and recycled.

From the foregoing, other alternate procedures and modifications of thevarious steps of the process of this invention will be obvious to thoseskilled in the art. Such obvious variations are considered to be withinthe proper scope of this invention.

What is claimed is:

1. Process which comprises contacting in the liquid phase a sourhydrocarbon feedstock containing sulfur compounds in an amount of notgreater than 2 weight percent with a crystalline zeolitic molecularsieve to adsorb selectively and isolate said sulfur compounds from saidhydrocarbon feedstock, thereafter desorbing sulfur compounds bycontacting the molecular sieve with a purge gas stream containingsufficient water vapor to load the molecular sieve with from 2 to 5weight-percent water at equilibrium, and thereafter contacting saidwater-containing molecular sieve with sour hydrocarbon feedstock in theliquid phase.

2. Process according to claim 1 wherein the temperature of the purge gasis between about 450 and about 700 F., the

pressure of the purge gas in contact with the zeolitic molecular sieveadsorbent is from about 15 to about p.s.i.a., and the dew point of thepurge gas stream is between 10 and 160 F.

3. Process according to claim 2 wherein the dew point of the purge gasstream is between 60 F. and F.

2. Process according to claim 1 wherein the temperature of the purge gasis between about 450* and about 700* F., the pressure of the purge gasin contact with the zeolitic molecular sieve adsorbent is from about 15to about 100 p.s.i.a., and the dew point of the purge gas stream isbetween 10* and 160* F.
 3. Process according to claim 2 wherein the dewpoint of the purge gas stream is between 60* F. and 140* F.